Part IA Linear Circuits and Device

Spice used for solving DC circuits problems (Examples Sheet 1)

The paragraphs below show how Spice may be used to solve awkward numerical
problems in DC circuit analysis. Both examples are taken from Examples
Sheet 1.

5. Determine the current in the 2W resistor in the circuit of Fig. 2 using
nodal voltage analysis. [L2]

The image below shows how the labelled circuit diagram (or schematic) can be
entered into the Spice package. Note that there are some differences from
the symbols to which you have become accustomed. If required, Spice offers
the flexibility to let you craft symbols to suit your own preferences.
Each component is uniquely labelled and a numbering system is used internally to
identify connections or nodes. The designer specifies values for
each component (note the resistor values - units are ohms - and current
sources). To determine the current in the 2-ohm resistor as required,
Spice lets the designer place a current monitor on the circuit. This will
report the current into the designated pin of the component R1 (which is Spice's
notion of the 2-ohm resistor).

Pressing the 'Simulate' button gives the following result.
The program has been instructed to plot the current as a function of time over a
period of one second. As all sources are DC, there is of course no
variation. The graph shows quite clearly how the current in the 2-ohm
resistor is 1 Ampere; the convention used by Spice ("current flowing into
the pin") shows that the (conventional) current flows left to right in the
circuit.

9. In the circuit of Fig. 5, find the current in X for V1 = 7V.

(HINT: Starting at the right hand side of the circuit, assume a current of 1A
flows in resistor X, and work out the potential drops back to the voltage
source. Then use scaling.)

Fig.
5

Again, the image below shows how the circuit schematic can be
entered into the Spice package. To determine the current in the resistor X
as required, a current monitor is placed on the circuit to
report the current into the designated pin of that component.

Pressing the 'Simulate' button gives the
result shown below. Again, for convenience, the program has been instructed to plot the current as a function of time.
All sources are DC, so no variation is seen. The graph shows that the current in
X is 125 mA, as expected

.

These
simple problems vastly understate Spice's enormous range of applications.
It really comes into its own with complicated circuits using linear and
non-linear devices, and signal waveforms of almost any arbitrary form.
Spice will determine for itself operating points and bias settings, and can be
programmed to show the output from a circuit under almost any conceivable
combination of inputs. We shall see some of these capabilities in
demonstrations later in the course.